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<title>Gcov-tool Intro (Using the GNU Compiler Collection (GCC))</title>

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<h3 class="section" id="Introduction-to-gcov_002dtool"><span>11.1 Introduction to <code class="command">gcov-tool</code><a class="copiable-link" href="#Introduction-to-gcov_002dtool"> &para;</a></span></h3>

<p><code class="command">gcov-tool</code> is an offline tool to process gcc&rsquo;s gcda profile files.
</p>
<p>Current gcov-tool supports the following functionalities:
</p>
<ul class="itemize mark-bullet">
<li>merge two sets of profiles with weights.

</li><li>read a stream of profiles with associated filenames and merge it with a set of
profiles with weights.

</li><li>read one set of profile and rewrite profile contents. One can scale or
normalize the count values.
</li></ul>

<p>Examples of the use cases for this tool are:
</p><ul class="itemize mark-bullet">
<li>Collect the profiles for different set of inputs, and use this tool to merge
them. One can specify the weight to factor in the relative importance of
each input.

</li><li>Collect profiles from target systems without a filesystem (freestanding
environments).  Merge the collected profiles with associated profiles
present on the host system.  One can specify the weight to factor in the
relative importance of each input.

</li><li>Rewrite the profile after removing a subset of the gcda files, while maintaining
the consistency of the summary and the histogram.

</li><li>It can also be used to debug or libgcov code as the tools shares the majority
code as the runtime library.
</li></ul>

<p>Note that for the merging operation, this profile generated offline may
contain slight different values from the online merged profile. Here are
a list of typical differences:
</p>
<ul class="itemize mark-bullet">
<li>histogram difference: This offline tool recomputes the histogram after merging
the counters. The resulting histogram, therefore, is precise. The online
merging does not have this capability &ndash; the histogram is merged from two
histograms and the result is an approximation.

</li><li>summary checksum difference: Summary checksum uses a CRC32 operation. The value
depends on the link list order of gcov-info objects. This order is different in
gcov-tool from that in the online merge. It&rsquo;s expected to have different
summary checksums. It does not really matter as the compiler does not use this
checksum anywhere.

</li><li>value profile counter values difference: Some counter values for value profile
are runtime dependent, like heap addresses. It&rsquo;s normal to see some difference
in these kind of counters.
</li></ul>


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